One of the key goals in exploring the properties of QCD matter is to determine the minimum value of the shear viscosity to entropy density ratio (η/*s*) for an ideal fluid. In heavy-ion collisions at the LHC, a quark-gluon plasma (QGP) is created, which is a state of hot and dense matter where quarks and gluons become deconfined. The plasma is formed at early times in the collisions and subsequently cools down to a temperature where the quarks and gluons cluster together into hadrons. The value of η/*s* is of particular interest, as weak coupling QCD and anti-de-Sitter/conformal field theory (AdS/CFT) theories predict different values. AdS/CFT is a technique from string theory that can be used to understand a strongly coupled system. The value of η/*s* implied by AdS/CFT is approximately 0.05–0.08, with calculations based on perturbative QCD techniques giving larger values.

The ALICE collaboration has recently released results of anisotropic-flow measurements from xenon–xenon (Xe–Xe) collisions at a per-nucleon centre of mass energy of 5.44 TeV, which offer additional constraints for the viscosity of the QGP. The anisotropic flow observed in a heavy-ion collision results from the spatial anisotropy of the initial collision zone, which is converted to momentum anisotropies via pressure gradients during the system̓s evolution. The magnitudes of momentum anisotropies are quantified by the harmonic coefficients ν_{n} of a Fourier expansion of the azimuthal distribution of particles; ν_{2} is generated by initial states with an elliptic shape, ν_{3} a triangular shape, and so on. The magnitude of ν_{n} depends not only on η/*s*, but also depends on the magnitude of the azimuthal asymmetries in the initial density distribution in the collisions. Comparing the new results from Xe–Xe collisions to those from lead–lead (Pb–Pb) collisions is expected to provide stronger constraints in the initial matter distribution, which will, in turn, provide a more precise determination of η/*s*.

The figure shows measurements of ν_{n} vs centrality for both Xe–Xe and Pb–Pb collisions. Centrality is a measure of the degree of overlap in heavy-ion collisions, where 0% corresponds to collisions that are head-on, and for 100% the heavy-ions do not overlap enough to interact. For mid-central collisions (20–70%), the second harmonic coefficients of the initial matter distributions are predicted to be very similar for Xe–Xe and Pb–Pb from various initial-state models. At the same centrality, however, the Xe–Xe system size is smaller than Pb–Pb and the impact of a finite η/*s* suppresses ν_{2} by 1/R, where R corresponds to the transverse size of the system. Therefore, ratios of Xe–Xe/Pb–Pb ν_{2} coefficients in the mid-centrality range could be directly sensitive to η/*s*, with larger values of η/*s* leading to a greater suppression of this ratio. When comparing our data to two different hydrodynamic models, which use parameters of η/*s* close to the values from AdS/CFT calculations, we find a good agreement with the data.

This shows that η/*s* is small, which implies a short mean-free path for the quarks and gluons in the QGP, or strong interactions. In central collisions, the ν_{2} in Xe–Xe collisions is larger than in Pb–Pb collisions. This is due to the ^{129}Xe nucleus not being exactly spherical and to larger fluctuations of the initial density distributions for the smaller Xe nucleus. The latter also gives rise to larger values of ν_{3} in the centrality range of 0–50%.

### Further reading

ALICE Collaboration 2018: arXiv:1805.01832.